This invention relates to microelectronic devices, and more particularly to a planar microelectronic triode and tetrode, and a method of fabricating the same.
A promising technology for use in high speed electronic systems is the vacuum microelectronic device, which in essence is a miniature vacuum tube that uses a cold emitter. In these microelectronic devices, electrons tunnel through a vacuum energy barrier whose width is determined by the electric field.
There are references in the art that discuss the design parameters for microelectronic devices. I. Brodie, "Physical Considerations in Vacuum Microelectronics Devices," Transactions on Electron Devices, Vol. 36, No. 11, Nov. 1989, pp. 2641-44, describes a vertical field-emission microtriode. For significant electron tunneling to take place at the tip of the emitter, the electric field at the tip must reach a relatively high strength (e.g., 1.times.10.sup.7 V/cm). To achieve such a high electric field, the emitters are provided with a relatively sharp tip (e.g., the point of a wedge, cone or pyramid shape). The emitter is placed relatively close to the extraction electrode. The closer the gap between emitter and extraction electrode, the lower the voltage needed to produce the requisite electric field strength and the less stringent the requirement for a vacuum. For a practical microtriode device operating in a 1 torr atmosphere, the distance between the anode and cathode should be 0.5 micrometers or less.
With microtriodes, another concern is device capacitance. Device capacitance is a function of the distance between the grid and the cathode, as well as the dimensions of the grid itself. In particular, the grid should be relatively small in size to reduce device capacitance.
At present, microelectronic devices have been constructed using conventional VLSI fabrication techniques (e.g., patterning and etching). For example, H. H. Busta et al., "Lateral Miniaturized Vacuum Devices," IEDM 89, pp. 533-36, describe a planar microtriode in which the anode, grid, and cathode (referred to as collector, gate and emitter, respectively) are fabricated using successive patterning and etching steps. In particular, the dimensions of the grid are defined by patterning and etching a conductive material. Similarly, Lee et al., U.S. Pat. No. 4,983,878, issued Jan. 8, 1991, describe a vertical microtriode in which the vertical positions of anode, grid and cathode are determined by the thickness of sacrificial layers, and the dimensions of the grid are determined by etching the polysilicon material of which the grid is formed (FIGS. 6 and 14). Tsukamoto et al., EP-A-0-416-558, describes a vertical microdiode that includes a cathode (electron emission element) and an anode (electrode 3008, FIGS. 3A and 3B), with both cathode and anode being formed using patterning steps to define dimensions, and with the vertical position of the anode relative to the cathode being defined by the thickness of a layer of material (insulating layer 3006, FIG. 3B).
Generally, devices fabricated using conventional VSLI techniques perform adequately. At present, however, conventional VLSI fabrication techniques have a resolution no better than about 0.5 micrometers, with a tolerance of about 10%). Practical microelectronic devices require a closer spacing of elements than 0.5 micrometers.
One method of achieving closer spacing of microelectronic elements is discussed in a copending U.S. patent application to the same inventor, Ser. No. 07/632,870, now U.S. Pat. No. 5,112,436, entitled, "Method of Forming Planar Vacuum Microelectronic Devices," filed Dec. 21, 1990, which describes a method of manufacturing a planar microelectronic device having a self aligned anode and cathode, the anode being formed from a sidewall spacer and the distance separating the anode and cathode being determined by the thickness of a sacrificial layer between the anode and cathode.